U.S. patent number 6,144,568 [Application Number 09/310,422] was granted by the patent office on 2000-11-07 for circuit arrangement for operating electrical lamps.
This patent grant is currently assigned to Patent-Treuhand-Gesellschaft fuer elektrische Gluehlampen mbH. Invention is credited to Felix Franck, Theodor Kiermeier.
United States Patent |
6,144,568 |
Franck , et al. |
November 7, 2000 |
Circuit arrangement for operating electrical lamps
Abstract
The invention relates to a circuit arrangement for operating
electrical lamps which encompasses an invertor (WR) with
oscillation build-up circuit (trigger generator), comprising diac
(DC1) and drive circuit. The drive circuit comprises a high-pass
filter, in particular an RC high-pass filter (R1, C1). As a result,
the trigger initiation is shifted closer to the zero crossing of
the supply voltage. This means, in turn, that a more sinusoidal
power supply current consumption with a smaller proportion of
harmonics is obtained. In addition, more stable lamp operation is
achieved in combination with power supply segment control
dimmers.
Inventors: |
Franck; Felix (Munich,
DE), Kiermeier; Theodor (Munich, DE) |
Assignee: |
Patent-Treuhand-Gesellschaft fuer
elektrische Gluehlampen mbH (Munich, DE)
|
Family
ID: |
7867586 |
Appl.
No.: |
09/310,422 |
Filed: |
May 12, 1999 |
Foreign Application Priority Data
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May 13, 1998 [DE] |
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198 21 351 |
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Current U.S.
Class: |
363/39; 315/224;
363/132; 323/905 |
Current CPC
Class: |
H05B
39/041 (20130101); H02M 7/53832 (20130101); Y02B
20/14 (20130101); Y10S 323/905 (20130101); Y02B
20/00 (20130101) |
Current International
Class: |
H05B
39/04 (20060101); H05B 39/00 (20060101); H02M
7/5383 (20060101); H02M 001/12 (); H02M
001/14 () |
Field of
Search: |
;363/37,39,34,124,132,89,98 ;315/129,224 ;323/905 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0647084B1 |
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Aug 1994 |
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EP |
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0682464A1 |
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May 1995 |
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EP |
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0682465A1 |
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May 1995 |
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EP |
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4011742C2 |
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Oct 1991 |
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DE |
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Other References
Douglas Thom, "Capacitor drops voltage with little heat for low
cost, low ltage power supply", Electronic Design, p. 148, Nov.
1975. .
Hirschmann, Hauenstein, Schaltnetzteile, Siemens AG, 1990, p. 102.
.
H.-J. Mayer, Stromversorgungen fuer die Praxis, Vogel Buchverlag,
Wurzburg, 1989, p. 115-116..
|
Primary Examiner: Wong; Peter S.
Assistant Examiner: Vu; Bao Q.
Attorney, Agent or Firm: Bessone; Carlo S.
Claims
What is claimed is:
1. A circuit arrangement for operating electrical lamps with
a free-running invertor (WR) with current feedback in a half- or
full-bridge circuit,
a coupling circuit, which is connected to the invertor (WR) and
adapts a high-frequency voltage of the invertor to electrical
lamp(s) (La; HG; LL) to be operated,
a trigger generator connected to the invertor (WR), with
a voltage-dependent switching element (DC1; DC1'),
a drive circuit for the voltage-dependent switching element (DC1;
DC1'), an output terminal (V) of the drive circuit being connected
to the invertor (WR) via the voltage-dependent switching element
(DC1; DC1'), and the voltage-dependent switching element (DC1;
DC1') turning on and, in the process, oscillation of the invertor
(WR) starting as soon as a potential of the junction point (V)
between drive circuit and voltage-dependent switching element (DC1;
DC1') exceeds a threshold value,
wherein the drive circuit essentially comprises a high-pass filter
circuit having a capacitor (C1; C1'), and in which an input
terminal (W) and the output terminal (V) of the high-pass filter
circuit are formed by a first terminal and a second terminal,
respectively, of the capacitor (C1; C1'), the high-pass filter
circuit additionally has a first resistor (R1; R1'), which is
connected to a junction point (V) between capacitor (C1; C1') and
the voltage-dependent switching element (DC1; DC1'), and to a
reference-ground potential of the circuit.
2. The circuit arrangement as claimed in claim 1, in which the
capacitor (C1; C1') has a capacitance which lies in a range from
approximately 2.2 nF to 22 nF.
3. The circuit arrangement as claimed in claim 2, in which the
high-pass filter circuit additionally has at least one further
resistor (R2a; R2b), which at least one further resistor (R2a; R2b)
is connected in parallel with the capacitor (C1; C1').
4. The circuit arrangement as claimed in claim 3, in which a ratio
of values of the at least one further resistor (R2a; R2b) to the
first resistor (R1) lies in a range from approximately 3 to 7.
5. The circuit arrangement as claimed in claim 1, in which the
high-pass filter circuit additionally has a second capacitor (C4),
which is connected to the junction point (V) between first
capacitor (C1) and voltage-dependent switching element (DC1), and
to the reference-ground potential of the circuit, and in which the
potential at the output terminal (V) follows a high-pass filter
characteristic with respect to the input terminal (W) of the drive
circuit.
6. The circuit arrangement as claimed in claim 3, in which a ratio
of capacitance C1 of the first capacitor (C1) to a sum C.sub.1
+C.sub.4 of capacitances of the first and second capacitor (C1,
C4), respectively, is greater than a ratio of a value R.sub.1 of
the first resistor (R1) to the sum R.sub.1 +R.sub.2 of values of
the first and at least one further resistor (R1, R2a, R2b),
respectively, such that the following relationship is fulfilled:
##EQU3## where, if appropriate, the value R.sub.2 designates a sum
of serial individual resistors (R2a, R2b).
7. The circuit arrangement as claimed in claim 1, in which a
current valve is connected between the junction point (V), which
connects the first capacitor (C1) to the voltage-dependent
switching element (DC1), and the reference-ground potential of the
circuit arrangement in such a way that the potential of the output
terminal (V) of the high-pass filter circuit does not fall below
the reference-ground potential of the circuit arrangement after the
triggering of the invertor.
8. The circuit arrangement as claimed in claim 1, in which the
input terminal (W) of the drive circuit is connected to a live
potential (E1, +U.sub.B) of the invertor or to another potential
which is suitable for triggering and has edges in a positive
direction.
9. The circuit arrangement as claimed in claim 1, in which the
voltage-dependent switching element of the trigger generator is a
diac (DC1; DC1'), a silicon bilateral switch (SBS) or a
programmable unit transistor (PUT).
10. The circuit arrangement as claimed in claim 1, in which the
circuit arrangement is an electronic converter suitable for the
operation of low-voltage incandescent halogen lamps, or an
electronic ballast suitable for the operation of fluorescent
lamps.
11. A system for operating electrical lamps with a phase segment
control dimmer and a circuit arrangement as claimed in claim 1.
Description
BACKGROUND OF THE INVENTION
The invention relates to a circuit arrangement for operating
electrical lamps. This type of circuit arrangement is used, in
particular, for operating low-voltage incandescent halogen lamps,
on the one hand, and discharge lamps, for example fluorescent
lamps, on the other hand, from an AC voltage power supply or a DC
voltage source. Circuit arrangements of this type are generally
called "electronic ballasts" (EBs) for the operation of discharge
lamps, while the designation "electronic transformer" or
"electronic converter" is customary for the operation of
low-voltage incandescent halogen lamps.
An essential component is a self-excited invertor with current
feedback, for example in a half-bridge or full-bridge circuit,
which invertor chops the low-frequency power supply voltage or the
DC voltage--which may also be pulsating--into a high-frequency
voltage. The latter is adapted to the electrical requirements of
the lamp(s) to be operated by means of a coupling circuit tuned to
the type of lamp.
In order to operate low-voltage incandescent halogen lamps, the
coupling circuit essentially comprises a power transformer which
transforms the high-frequency voltage to the low voltage of the
incandescent halogen lamps to be used, e.g. 6, 12 or 24 V.
Therefore, such circuit arrangements are also referred to as
electronic converters. Their detailed method of operation is
disclosed for example in EP-V 264 765 and DE-A 40 11 742.
In order to operate discharge lamps, for example fluorescent lamps,
the bridge transistors are followed by a ballast inductor which
limits the lamp current. In order to ignite the discharge, the
coupling circuit may also comprise a capacitor which is connected
in parallel with the electrodes of the discharge lamp and is
operated in resonance with the ballast inductor. The method of
operation of such electronic ballasts is explained in more detail
for example in DE-C 29 41 822 and DE-A 38 05 510.
In both cases, the invertor is controlled by an output current
component that is fed back. As a result, in order to start the
oscillation of the invertor for the first time (see e.g. EP 0 682
464 A1), for example immediately after the switching on of the
supply voltage, and also in order to restart the oscillation after
each zero crossing of the power supply voltage (see e.g. EP 0 682
465 A1 or EP 0 647 084 A1), in particular in electronic converters,
a control pulse is necessary to initiate the HF oscillation of the
invertor. Electronic converters or ballasts usually contain an
oscillation build-up circuit, also called a start or trigger
generator, which performs this task.
In the simplest case, the trigger generator essentially comprises a
charging capacitor, a charging resistor and a voltage-dependent
switching element, for example a diac. The charging capacitor is
initially charged via a charging resistor. When the voltage of the
charging capacitor reaches the threshold value of the
voltage-dependent switching element--the triggering voltage of the
diac in the example--the switching element turns on and the
high-frequency oscillation of the invertor starts. During operation
of the invertor, on the other hand, it is necessary to prevent the
generation of a trigger signal that interferes with the
high-frequency oscillation through the voltage-dependent switching
element.
PRIOR ART
EP 0 682 464 A1 discloses a circuit arrangement for operating
electrical lamps with an invertor and a trigger generator. The
trigger generator comprises a resistor, a charging capacitor, a
diac and a controllable discharge resistor. The controllable
discharge resistor prevents the production of trigger pulses while
the half-bridge oscillates.
The charging capacitor of the trigger generator is charged via the
serially connected resistor. The controllable discharge resistor is
realized by an NPN transistor, for example. Its collector-emitter
path is connected in parallel with the charging capacitor. In one
exemplary embodiment, this collector-emitter path is turned on,
i.e. acquires a low impedance, in synchronism with the
collector-emitter path of a bridge transistor of the invertor, with
the result that the charging capacitor can be discharged via the
collector-emitter path of the discharge transistor. As a result, an
undesirable trigger pulse is reliably prevented from being
superposed on the control voltage for the bridge transistor in a
simple manner.
What is disadvantageous about this kind of trigger pulse generation
is the fact that the signal which drives the diac reacts relatively
slowly to changes in the supply voltage, for example in the course
of switching on or reswitching on after a malfunction of the
circuit arrangement. Particularly with a pulsed supply voltage, as
is supplied, if appropriate, by bridge rectifiers in the input of
circuit arrangements of this type, rapid triggering is
indispensable for a power supply current consumption which is as
sinusoidal as possible, i.e. having a minimum proportion of
harmonics. The maximum permissible proportion of harmonics is
regulated by corresponding standards (IEC 1000-3-2).
A further problem may arise when so-called phase gating and
chopping dimmers are used. In this case, a so-called flicker effect
can occur under certain circumstances. The lamp operated from the
output of the circuit arrangement is alight with temporal
interruptions such that the human eye perceives the noncontinuous
light emission in a disturbing manner.
The main reason for the abovementioned problems may be seen in the
response of the circuitry that drives the diac. Specifically, the
circuitry in EP 0 682 464 A1 is an RC low-pass filter which
transfers a signal change at its input to its output with a delay.
As a result, a step function at the input, for example, is
transferred to the output in a manner such that it is "degraded" to
a greater or lesser extent.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a circuit
arrangement for operating electrical lamps with an improved trigger
generator.
The invention proposes using a high-pass filter as drive circuit
for the voltage-controlled switching element of the trigger
generator. By means of the high-pass filter, a change at the input
of the drive circuit is transferred ideally immediately, i.e.
without delay, to the output of the drive circuit and consequently
to the switching element. When the trigger threshold is reached,
finally, the switching element applies a trigger signal to the
invertor, whereupon the latter starts to build up oscillations or
build up oscillations again.
A series of advantages are achieved by this measure. For the case
where the supply voltage is a sinusoidal AC voltage or pulsating DC
voltage, the circuit according to the invention causes the
initiation of the respective trigger signal to be shifted closer to
each zero crossing or zero value of the supply voltage. As a
result, a more sinusoidal power supply current consumption than in
the prior art is obtained during power supply operation.
Furthermore, in combination with a phase gating dimmer, stable,
flicker-free lamp operation is ensured even in the dimming mode
with small power consumptions, since the trigger pulses follow the
abrupt voltage rises of the phase gating dimmers virtually without
any delay. The resulting advantage for phase chopping dimmers is
that a smaller lower limit of the setting of the dimming mode can
be achieved in comparison with the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail below using a number
of exemplary embodiments. In the figures:
FIG. 1 shows a basic circuit diagram of the circuit arrangement
according to the invention with a trigger generator having a
particularly simple structure,
FIG. 2 shows a variant with respect to FIG. 1,
FIG. 3 shows a further variant with respect to FIG. 1,
FIG. 4 shows another further variant with respect to FIG. 1,
FIG. 5 shows a simplified circuit diagram of an inventive
electronic converter for low-voltage incandescent halogen lamps,
and
FIG. 6 shows a simplified circuit diagram of an inventive
electronic ballast for fluorescent lamps.
BEST MODE FOR CARRYING OUT THE INVENTION
In order to elucidate the invention further, reference is made
below to FIG. 1, which shows a circuit arrangement according to the
invention with an invertor WR and a load La connected thereto, and
also with a particularly simple design of the high-pass filter
circuit in a schematic basic illustration. In this case, the
high-pass filter circuit comprises a capacitor C1 connected
serially between the voltage-dependent switching element, in this
case a diac DC1, and the live input terminal E1 of the circuit
arrangement. In other words, the input terminal W of the high-pass
filter circuit is connected to the live input terminal E1 of the
circuit arrangement, and the output terminal V of the high-pass
filter circuit is connected to the voltage-dependent switching
element. In principle, the input terminal W of the high-pass filter
circuit may also be connected to another potential which is
suitable for triggering and has principally unambiguous edges in
the positive direction. The second input terminal E2 of the circuit
arrangement in this case defines the reference-ground potential.
The capacitor C1 acts as a driver capacitor which transfers the
changes in the potential at the input W of this purely
differentiating element directly to the voltage-dependent switching
element DC1.
A further advantage that may be emphasized--in addition to the
rapid trigger initiation--is the minimal number of components,
specifically just a single one, for the drive circuit for the
voltage-dependent switching element.
This is because, in contrast to the prior art (not illustrated), in
this case there is no charging capacitor C which must first be
charged via a charging resistor R and reach the threshold value for
the triggering of the diac with the time constant
.tau.=R.multidot.C. As a result, the disadvantageous effect of
delayed trigger initiation as explained above in the context of the
prior art is avoided in the case of the circuit according to the
invention.
In one variant (see FIG. 2), the drive circuit from FIG. 1 is
supplemented to form an RC high-pass filter. For this purpose, a
resistor R1 is connected to the junction point between capacitor C1
and diac DC1, on the one hand, and the reference-ground potential
E2 of the circuit, on the other hand. In this case, then, the
output V of the high-pass filter circuit is formed by the junction
point between capacitor C1 and resistor R1. The resistor R1 offers
a further degree of freedom for influencing the level and the shape
of the voltage profile at the junction point V and hence the
switching behavior of the diac DC1. Furthermore, this variant is
suitable for repeated triggering, since the initial charge in the
capacitor C1 is reestablished each time via the resistor R1 and the
invertor as long as the invertor operates.
The capacitance of the capacitor C1 of the high-pass filter circuit
typically lies in the range from approximately 2.2 nF to 22 nF.
In a further variant (see FIG. 3), at least one further resistor R2
is connected in parallel with the capacitor C1, i.e. the resistor
R2 is connected, on the one hand, to a first terminal of the
capacitor C1 at the input terminal W of the high-pass filter
circuit and, on the other hand, to the second terminal of the
capacitor C1 at the junction point V--the output terminal of the
high-pass filter circuit. For practical considerations, it may also
be expedient to connect two or even more resistors in parallel with
the capacitor C1 in order to divide the voltage between more than
one resistor and hence to increase the voltage endurance of the
circuitry. The at least one further resistor R2 acts, together with
the first resistor, as a resistive voltage divider connected in
parallel with the input of the circuit arrangement. The center tap
of this voltage divider corresponds to the junction point V, i.e.
to the output terminal of the high-pass filter circuit, and in this
way drives the diac as well.
As a result, it is possible also to utilize low-frequency voltage
components or DC voltage components of the input voltage in the
driving of the voltage-dependent switching element (diac DC1). A
further aspect is that the resistive conductivity of the circuit
arrangement is maintained even when the invertor is not
oscillating. This is important particularly In combination with
phase chopping dimmers, in order to ensure the inherent supply
thereof.
A further advantage is that undervoltage identification is also
realized, at the same time, by means of the resistive voltage
divider. Specifically, in order that the invertor can be started in
the first place, the input voltage of the drive circuit, i.e. the
voltage between the input terminal W of the high-pass filter and
the reference-ground potential, must be greater than the trigger
threshold of the diac DC1 multiplied by the duty ratio of the
resistive voltage divider. This property is very important for the
operation of the circuit arrangement on phase chopping dimmers,
since this effectively prevents an uncontrolled restart of the
invertor after the chopping of the power supply voltage by the
dimmer.
In principle, the first resistor is dispensable in this case, too,
if it is possible to dispense with operation with phase chopping
dimmers and with prevention of undesired starting in the event of
undervoltage.
The ratio of the values of the at least one further resistor to
that of the first resistor, that is to say R2 to R1, typically lies
in the range from approximately 3 to 7, where R2 denotes, if
appropriate, the sum of a plurality of serial resistor
elements.
In order to prevent the potential of the junction point V from
falling below the reference-ground potential after the triggering
of the invertor, as a result of which the next trigger instant
would be delayed in an uncontrolled manner, a current valve, for
example a diode, may be connected between the junction point V,
which connects the capacitor to the voltage-dependent switching
element, and the reference-ground potential of the circuit
arrangement.
In a further variant (see FIG. 4), a second capacitor C4 is
connected in parallel with the first resistor R1. Said second
capacitor is used to attenuate the high-pass filter characteristic
for adaptation reasons and also, in particular, to increase the
robustness of the circuit arrangement with respect to overvoltage
pulses at the supply end. The value C.sub.4 of the second capacitor
C4 is dimensioned such that the drive circuit retains its high-pass
filter characteristic and does not degenerate into an undesirable
low-pass filter. In the present case--connection in parallel of a
resistive voltage divider (R1, R2) and of a capacitive voltage
divider (C1, C4) with a common tap (=junction point V)--this means
that the ratio of the capacitance C.sub.1 of the first (high-pass
filter) capacitor C1 to the sum C.sub.1 +C.sub.4 of the
capacitances of the two capacitors C1, C4, that is to say C.sub.1
/(C.sub.1 +C.sub.4), is greater than the ratio of the value R.sub.1
of the first resistor R1 to the sum R.sub.1 +R.sub.2 of the
resistors R1, R2, that is to say R.sub.1 /(R.sub.1 +R.sub.2). This
is because, as may be shown, the influence of the capacitive
component at the tap of the two coupled voltage dividers, i.e. the
output V of the drive circuit, is predominant in the event of
signal changes at the input W of the drive circuit.
The principle behind the invention as explained above in general
terms using an invertor with a connected load can expediently be
used both for electronic transformers and for EBs, for which,
therefore, protection is expressly claimed in each case.
The circuit arrangement may, in particular for power supply
operation, additionally be supplemented by a rectifier circuit.
Furthermore, the circuit arrangement may additionally have, in a
supplementary fashion, a filter circuit which protects the power
supply against high-frequency interference signals of the
half-bridge converter in a manner known per se and comprises for
example an interference-suppression inductor and one or more
capacitors. Finally, the circuit arrangement may also additionally
have a protection device affording protection against malfunctions,
overload and/or short circuit or the like.
Furthermore, protection is also claimed for a system for operating
electrical lamps with the circuit arrangement according to the
invention and a dimmer, in particular a phase gating or chopping
dimmer, since it is precisely in combination with a dimmer that the
specific advantages of the circuit arrangement according to the
invention become especially evident.
FIG. 5 shows a simplified circuit diagram of an electronic
converter for low-voltage incandescent halogen lamps. It comprises
the function blocks of radio interference suppression FE, rectifier
GR, self-excited half-bridge converter with current feedback and
trigger generator. The half-bridge converter comprises two
half-bridge transistors T1, T2, two half-bridge capacitors C2, C3,
a control transformer RKA-RKC for the current feedback and also a
power transformer TR. The trigger generator is formed by a diac DC1
and a drive circuit, comprising a first resistor R1 and also a
serial pair of resistors R2a and R2b, a first capacitor C1 and a
second capacitor C4 and also a diode D1. An NPN transistor T3 with
an associated base series resistor R3 prevents the production of
trigger pulses while the half-bridge oscillates.
The method of operation of the circuit arrangement shown in FIG. 5
is explained in more detail below. The radio interference
suppression FE protects the power supply against high-frequency
interference signals of the half-bridge converter in a manner known
per se and comprises for example an interference-suppression
inductor and one or more capacitors (see e.g. H.-J. Meyer,
"Stromversorgungen fur die Praxis (Practical power supplies)",
Vogel Buchverlag, Wurzburg, 1989, pp. 115-116).
The rectifier GR comprises a diode full-bridge (see e.g. W.
Hirschmann and A. Hauenstein, "Schaltnetzteile (Switched-mode power
supplies)", Siemens A G, 1990, p. 102) and converts the AC voltage
of the power supply into a pulsating DC voltage +U.sub.B. The
negative pole of the rectifier is the reference-ground potential in
the following text.
The two bridge transistors T1, T2 are alternately turned on by the
voltage signals of the two secondary windings RKB and RKC,
respectively, of the control transformer RKA-RKC. They thus close
the electric circuit via the primary windings of control
transformer RKA-RKC and power transformer TR, and also via the two
bridge capacitors C2 and C3, respectively. The secondary winding of
the power transformer TR is connected to a 12 V incandescent
halogen lamp.
The first capacitor C1 and the first resistor R1 of the drive
circuit are connected in series. The free end of the first
capacitor C1 is connected to the pulsating DC voltage +U.sub.B and
the free end of the first resistor R1 is connected to the
reference-ground potential. The junction point V between first
capacitor C1 and first resistor R1 consequently acts as the output
of a high-pass filter circuit which drives the diac DC1. For this
purpose, the junction point V is connected to the diac DC1. The
discharge diode D1 is connected in parallel with the first resistor
R1. The serial pair of resistors R2a, R2b is connected in parallel
with the first capacitor C1. The pair of resistors R2a, R2b acts,
together with the first resistor R1, as a resistive voltage divider
connected between the pulsating DC voltage +U.sub.B and the
reference-ground potential. The center tap of this resistive
voltage divider R1 and R2a, R2b corresponds to the junction point V
and, in this way, likewise drives the diac DC1. At the same time,
the series circuit of R2a, R2b and R1 acts as conductance and
consequently increases the resistive conductivity of the circuit.
This is highly beneficial to the operation of the circuit
arrangement on a phase chopping dimmer. In order to attenuate the
high-pass filter characteristic for adaptation reasons and, in
particular, to increase the robustness of the circuit arrangement
with respect to overvoltage pulses at the supply end, the second
capacitor C4 is connected in parallel with the first resistor
R1.
The collector-emitter path of the discharge transistor T3 is
connected in parallel with R1 and D1. Said collector-emitter path
is turned on, i.e. acquires a low impedance, in synchronism with
the collector-emitter path of the bridge transistor T2--except for
a possible slight phase shift due to different switching times of
the transistors T2 and T3--, with the result that the second
capacitor C4 is discharged via the collector-emitter path of the
discharge transistor T3 and, at the same time, the first capacitor
C1 can be charged further, with the result that the potential at
the junction point V reliably remains below the trigger threshold
of the trigger element DC1. For the purpose of synchronization, the
base terminals of the two transistors T2, T3 are connected to one
another via the series resistor R3, with the result that both the
bridge transistor T2 and the discharge transistor T3 are driven by
the control voltage of the secondary winding RKC of the control
transformer RKA-RKC. As a result, an undesirable trigger pulse is
reliably prevented from being superposed on the control voltage for
the bridge transistor T2 in a simple manner. The resistor R3 is
used, on the one hand, to protect the transistor T3 against
overloading. On the other hand, as a result of its dimensioning,
the trigger parallel current via the collector-emitter path of the
transistor T3 is limited in a targeted manner such that sufficient
initial triggering of the half-bridge converter is ensured via the
actual trigger path, comprising the diac DC1 and the base-emitter
junction of the half-bridge transistor T2. As desired, this means
that a trigger signal is generated only when the HF oscillation Of
the half-bridge terminates, for example as a result of the power
supply voltage being switched off--if appropriate also only
temporarily.
A list of components for a concrete exemplary embodiment is
specified in the Table.
The following critical values for the action of the drive circuit
from FIG. 5 as a high-pass filter result from the list of
components: ##EQU1## It is evident from this that the relationship
##EQU2## introduced in the general part of the description is
fulfilled in this case and, consequently, the drive circuit from
FIG. 5 acts as a high-pass filter as desired.
FIG. 6 shows a simplified circuit diagram of an inventive
electronic ballast for fluorescent lamps. The filter circuit FE'
corresponds, in principle, to the filter circuit FE described in
FIG. 1. The rectifier GR' contains a smoothing capacitor in
addition to a diode full-bridge. Said smoothing capacitor limits
the degree of modulation of the rectified supply voltage
(modulation frequency 100 Hz) to a value which does not cause any
interfering modulation of the light intensity of the fluorescent
lamp LL.
The invertor--which in this case comprises a current feedback
half-bridge with the two bridge capacitors T1' and T2', the two
bridge capacitors C2' and C3' and the control transformer
RKA'-RKC'--and also the trigger generator, comprising R1', R2a',
R2b', C1' and DC1', correspond, in terms of their fundamental
methods of operation, to those already described in FIG. 5. The
second capacitor of the drive circuit for the diac DC1' is
dispensed with here, in contrast to FIG. 5.
Furthermore, in contrast to FIG. 5, the discharge diode of the
trigger generator is dispensed with. This is because said discharge
diode is dispensable owing to the smoothing capacitor in the
rectifier GR'. The smoothing capacitor prevents the potential at
the junction point V from falling below the reference-ground
potential.
The two secondary windings RKB', RKC' of the control transformer
RKA'-RKC' are connected to the base terminals of the two bridge
transistors T1' and T2', respectively, via the series resistors R4
and R5. The controllable discharge resistor is in this case
realized by the FET T3'. The latter is a normally off,
enhancement-mode n channel MOS-FET. It is connected in a manner
corresponding to the NPN transistor T3 in FIG. 4 and is likewise
driven essentially in synchronism with the bridge transistor T2' by
the control signal of the secondary winding RKC' of the control
transformer RKA'-RKC' via the series resistor R2'. The inductor L1
stabilizes the current of the fluorescent lamp LL. Said inductor
forms a resonant circuit with the capacitor C4. The dimensioning of
said resonant circuit is chosen such that it has a high quality
factor during the ignition phase and its resonant frequency is near
the operating frequency of the invertor. The high ignition voltages
necessary for igniting the fluorescent lamp LL are achieved as a
result of this.
The invention is not limited by the embodiments specified. In
particular, individual features illustrated in the figures may also
be essential to the invention in other combinations.
TABLE ______________________________________ List of components for
an exemplary embodiment in accordance with the simplified circuit
diagram from FIG. 5. ______________________________________ R1 100
k.OMEGA. R2a + R2b 360 k.OMEGA. R3 1 k.OMEGA. C1 8.2 nF; 400 V C2,
C3 15 nF; 400 V C4 10 nF; 63 V D1 1N4148 DC1 Diac 32 V RKA-RKC R
10/6/4; 5:5:1 turns TR EF 25/7.5; 63:7 turns T1, T2 BUL38D T3
BC850C HG Incandescent halogen lamp 12 V, 50 W
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